Many natural alkaloids and related analogs bind to specific opioid receptors and elicit an analgesic response similar to classic narcotic opiates. Many different types of opioid receptors have been shown to coexist in higher animals. For example, see W. Martin et at., J. Pharmacol. Exp. Ther. 197, p. 517 (1975); and J. Lord et al., Nature (London), 257, p.495 (1977). Three different types of opioid receptors have been identified. The first, δ, shows a differentiating affinity for enkephalin-like peptides. The second, μ, shows enhanced selectivity for morphine and other polycyclic alkaloids. The third, κ exhibits equal affinity for either group of the above ligands and preferential affinity for dynorphin. In general, the μ receptors seem to be more involved with analgesic effects. The δ receptors appear to deal with behavioral effects, although the δ and the κ receptors may also mediate analgesia.
Each opioid receptor, when coupled with an opiate, causes a specific biological response unique to that type of receptor. When an opiate activates more than one receptor, the biological response for each receptor is affected, thereby producing side effects. The less specific and selective an opiate may be, the greater the chance of causing increased side effects by the administration of the opiate.
Opiates can cause serious and potentially fatal side effects. Side effects such as respiratory depression, tolerance, physical dependence capacity, and precipitated withdrawal syndrome are caused by nonspecific interactions with central nervous system receptors. See K. Budd, In International Encyclopedia of Pharmacology and Therapeutics N. E. Williams and H. Wilkinson, Eds., Pergammon: (Oxford), 112, p.51 (1983). It is therefore an object of the present invention to provide compounds having analgesic effects but having as few side-effects as possible.